When Sven Lindqvist was a boy, he saw a photograph of what British troops found in Buchenwald. "The heaps of naked bodies, people who obviously had been starving and worked to death, burned in one's eyes," he says. A few years later, aged 17, he read Heart of Darkness. In a shady grove, Marlow discovers black Africans worked shamelessly by white colonialists lying "in all the attitudes of pain, abandonment and despair … They were dying slowly - it was very clear."

"These two pictures moved into each other for me," says Lindqvist. "I saw them as one image of extermination." When he first read Heart of Darkness, Lindqvist took Conrad to be prophesying what was coming rather than writing about what he had seen. But later he became convinced that the kind of racist extermination Conrad witnessed prefigured that of Hitler.

In the Swede's favourite and perhaps best book, Exterminate All the Brutes, first published in 1992, Lindqvist writes that "the Germans have been made sole scapegoats of extermination that are actually a common European heritage … The ideas he [Hitler] and all other western people in his childhood breathed were soaked in the conviction that imperialism was a biologically necessary process, which, according to the laws of nature, leads to the inevitable destruction of the lower races. It was a conviction which had already cost millions of lives before Hitler provided his highly personal application."

This, to put it mildly, is a controversial view. For example, Jewish philosopher Steven T Katz in his 1994 book The Holocaust in Historical Context argued for the "phenomenological uniqueness of the Holocaust". And during the historikerstreit (historians' quarrel) in 1980s Germany, Ernst Nolte provoked fury among fellow intellectuals with his contention that the Holocaust was Hitler's "distorted copy" of Stalin's extermination of the Kulaks. For Lindqvist, Nolte's mistake was to look east for Hitler's inspiration. He should have looked west. Lindqvist writes: "When Hitler sought Lebensraum in the east it was a continental equivalent of the British empire."

"There are very substantial differences between the Holocaust and other genocides," he says. "These other genocides can still be reasons for, and causes of, the Holocaust." In Exterminate All the Brutes Lindqvist suggests that we have airbrushed our past: "We do not want to remember. We want genocide to have begun and ended with nazism. That is what is most comforting." Now 20 years after the first Swedish edition, he says with some pride, that thanks to his book there is a thriving field of historical research on the effect of colonial atrocities on Nazi crimes.

"Exterminate all the brutes" are the words of Kurtz, the ivory-trading murderer and imperialist demi-god whom Marlow travelled up the Congo river to meet. "Why," writes Lindqvist, "did Kurtz end his report on the civilising task of the white man in Africa with these words? Why did Conrad make them stand out as a summary of all the high-flown rhetoric on Europe's responsibilities to the peoples of other continents?"

Lindqvist tried to answer these questions in a book that is extraordinary for not being straightforward historical text but travel diary. He wrote it while crossing the Sahara on buses and, at the same time, journeying through the history of extermination. Iain Sinclair once divided writers into pods (writers who stay in their studies) and peds (writers who roam and walk) – Lindqvist is decidedly of the latter temperament. On the road, he considers what Darwin wrote in The Descent of Man: "The civilised races of man will almost certainly exterminate and replace throughout the world the savage races" – words used by him to justify colonial extermination of native peoples. Lindqvist reflects on what Darwin's cousin, Francis Galton, and later European racial biologists, would argue to justify the genocide of non-whites across the world. He steeps himself in German thinkers whose genocidal texts provided sham-intellectual justification for murdering 6 million Jews.

Lindqvist has often worked this way and, in 30 books written over the past half century, has developed a literary form flexible enough for him to travel in time as much as space, combining the personal and the political, mingling historical investigation, travel and literary reportage and – increasingly – fierce polemic. He writes in terse numbered paragraphs, following Nietzsche's aphoristic style. He'll leap from an account of a dream into a passage of shattering political rage. In the penultimate paragraph 168 of Exterminate All the Brutes, for instance, he writes: "Everywhere in the world where knowledge is being suppressed, knowledge that, if it were made known, would shatter our image of the world and force us to question ourselves – everywhere there Heart of Darkness is being enacted."

Lindqvist and I are meeting at the home of Sigrid Rausing, heiress to the Tetra Pak millions who has deployed her money to bankroll Granta, publisher of Lindqvist's books in English. To celebrate his 80th birthday this year, Granta is reissuing three volumes of his best books and publishing one in English for the first time. Rausing's home is a little outpost of Sweden: Lindqvist sips tea from a Moomin Troll mug; on the walls are elegant Swedish artworks including a tapestry depicting Biblical scenes.

He points at his twitching leg, which I thought a sign of his irritation at my inept questions. But no: "I've had Parkinson's for eight years. I discovered it when I was driving a car. An extremely small difference in pressure on the pedal changes speed a great deal. I didn't think it was a fault of mine that my car kept jumping, I thought it was the fault of construction of all cars. It turned out I had Parkinson's. Now I don't drive."

Parkinson's and age have curbed lifelong wanderlust. "Travelling at my age, even if you don't have Parkinson's, is not good idea." He was born in Stockholm in 1932 and was the first in his family to leave Sweden. "Nobody I knew left except an Aga engineer who had left briefly on technical business." He departed his homeland in 1947 for the first time to study French and German in Switzerland. "I travelled through Germany by train. Everywhere was destroyed. Germans were running along the train crying for bread. Nothing left of whole cities. That was Europe."

Four decades later, those experiences would figure in his most critically feted book in English, A History of Bombing(first published in Swedish in 1999 and in English in 2001). Lindqvist wrote of how the evils Europeans perpetrated in their colonies prefigured the violence they would commit against each other at home: "Once a commander has set fire to an African capital," he wrote, "might he have learned to burn Paris or Berlin? The type of war that Europe had allowed itself to wage against three-quarters of humanity – was that what came back to haunt us in the 20th century?"

The fractured structure of that book – Lindqvist suggests multiple entry paths through its numbered paragraphs, breaking each journey through the text into fragments – is emblematic of the disruption caused by the aerial bomb. Ever since 1 November 1911 when an Italian lieutenant threw a grenade from his delicate plane, the dream of exterminating from the air with minimal risk to bombers has been realised, he argues. The destroyer of German cities, Arthur "Bomber" Harris scored his "first civilian hits", in 1919 in Kabul, writes Lindqvist. He argues that the aerial bombing that marked the second world war and now has its culmination in unmanned drones controlled from Nevada as they bomb Afghan villages, had its roots in the colonial policy of exterminating "savages" from the air.

In a chilling afterward to the 2012 edition he writes that "The drone war is the unmanned opposite of the mass armies of the Great War. Convenient – so long as the other side hasn't got one." But, he reminds us, drones are dirt cheap. "What if the drone becomes every man's or at least every state's weapon? Will it not be too easy to wage war, if all you risk are machines, not soldiers?" Perhaps in the future Nevada will be bombed by drones controlled from Afghan villages.

Lindqvist suggests that the British should learn from Germans in reflecting on their misdeeds. "The Germans are much more thorough than other peoples in researching their crimes and atrocities. The bombing story really started with the RAF in India, Afghanistan, Iraq, Kenya, Darfur, Egypt, but it is a story scarcely told."

The Sven Lindqvist of such fiercely polemical works as these is remote from the teenager who first became obsessed with literature. "I wasn't politically engaged in my youth. I was more engaged by Nietzsche and Goethe. My Swedish master was Vilhelm Ekelund, a poet, writer of aphorisms and follower of Nietzsche." In his 20s he was an iconoclastic aesthete, who learned Chinese with the great Swedish sinologist, Bernard Karlgren, in Stockholm, not out of political commitment to Mao's recent revolution, but out of love for a venerable culture of grace and simplicity which, he thought, represented the blissful antithesis of the consumerist west.

One of Lindqvist's earliest books was called Advertising Is Lethal. "It's a critique of advertising and marketing. I wrote it when I had recently married and we had to buy everything to make a household. I revolted against the idea that I would have to do work I didn't want to do just in order to pay instalments on a sofa. I'd much rather not have a sofa and use my time to do some reading, writing, loving and experiencing life."

The initial failure of that book – "it got enormously bad reviews because it was so one-sided" – made Lindqvist think that changing a corrupt society was impossible. He left his homeland in the early 60s to spend two years in China, steeping himself in its language and calligraphy. How odd that you weren't there to engage with socialist revolution. "It seemed strange in those days to be interested in Chinese literature and culture. Nobody thought the Chinese would become economically powerful, nor that the Chinese language would become à la mode and needed by big business."

That aestheticised self, steeped in Chinese culture, seeking mystic calm and refuge from a degraded west, was submitted to something very like Maoist self-criticism in his 1967 book The Myth of Wu Tao-Tzu, possibly his finest and certainly most philosophically profound work which Granta is to publish in English for the first time in August. The myth is that of the eponymous artist who stepped into his painting as the culmination of his work and to elude quotidian reality. The book later follows Lindqvist as he journeys through China, India and Afghanistan, experiencing a political awakening. During the book's journey, he challenges those who have used art as escape from an insufferable world – in particular Herman Hesse, whom he admired. He rejects too his own literary style: "My friends are perhaps right when they say that ever since then my writing has lost its beauty and harmony. What are beauty and harmony when you are running for dear life between crumbling walls? I am beginning to believe my artistic endeavours have been a mistake."

He rounds on Hesse, who wrote his iconoclastic novel The Glass Bead Game, which Lindqvist adored, between 1931 and 1943. The admirer came to see Hesse's rarefied project as despicable, a withdrawal from an intolerable Europe when engaging with nazism was necessary. He wrote: "Is it the function of art to make mass graves banal? Is it the task of thought to make starvation uninteresting? Spiritual happiness that makes the world irrelevant will also make suffering, oppression and extermination irrelevant."

Like Axel Heyst, the Swedish protagonist of Conrad's Victory, Lindqvist was dragged back into the vile real world from mere renunciation. It became his task to use his literary art in an opposite way to Hesse, even though he despaired of what literature might achieve or of the capacity of rich Europeans to change. "We have created a lifestyle which makes injustice permanent and inescapable … Is the conclusion clear?" he howls towards the end of The Myth of Wu Tao-Tzu. "We have to become poor again. Or uphold our privileges with violence. No people, even less a continent, has voluntarily chosen poverty. Nor are there any prospects of us being about to."

Subsequently, Lindqvist wrote increasingly furious books his targets did not want to read. The apogee, for me, is his book Terra Nullius, a 2005 Australia travelogue that indicts Britons and white Australians for terrible abuses such as the transportation of Aborigine women to the chillingly named Isle of the Dead where they were given inappropriate and often fatal syphilis treatment, and the extensive forced separation of "half-blood" children from their families to prison-like camps. How was that received in Australia? "It seems as if it was largely ignored," he says. "An Australian [Peter Conrad] wrote a very superficial article in the Observer. I think his judgment doomed the book for Australians – as though there was nothing to discuss."

One day, aged 53, Lindqvist was sitting in a Stockholm sauna and fell to chatting with a muscle-bound skinhead who convinced him that he ought to try bodybuilding. The relatively weedy intellectual decided to attend the local gym. The result was an epiphany as radical as his awakening as a politically engaged writer. Bench Press(Sweden 1988, UK 2003) traced Lindqvist doing to his body what Ezra Pound recommended the writer do to text: "A writer's work can have definition. Pound called it being sparing with the mortar. The less mortar, the sharper the definition. That's what I've always striven for." Like Arnold Schwarzenegger or Yukio Mishima, he realised himself by pounding his body, finding bliss in pumping iron and remembering thereby long-repressed childhood dreams, including ones about the Sahara and the longings he had to go alone into its empty spaces.

He fulfilled that dream by travelling across the Sahara in his mid 50s. He had just broken up from Cecilia, his wife of 33 years, with whom he had made a trip through Latin America in the 1960s with their three-year-old son, Aron. "As a child I dreamt about fire eaters and well divers, about sandstorms and desert lakes. I planned a great journey in the Sahara. Now I am here. I am alone," he later wrote. That loneliness was, like bodybuilding, a welcome challenge.

Lindqvist now lives with his second wife Agneta Stark, an economist, in a flat in Stockholm. His solitary wanderings are, perhaps, over. That said, when I met him he had travelled to London to hole up alone in a boat suspended above the Queen Elizabeth Hall. The art commissioners Artangel had invited him to write a 2,000-word text while resident there for a week. It was a freighted project: the former traveller held in suspended animation, alone with his laptop.

The text he wrote, entitled "A London Address", hardly disappoints. Inspired by Jack London's 1903 book People of the Abyss about how imperial London treated its East End poor, Lindqvist reflects on the same subject a century on as the capital of imperial shame postures and struts Olympian. "We have created a world where … capital breeds capital with very little need for the Eastenders of the world. Tell me what will happen when the majority of mankind has become technologically superfluous." We have, if Lindvist is right, brought the heart of darkness home.